The present invention relates to a highly lubricant grease composition free from the problem of forming a black denaturated matter during use.
Needless to say, lubricants such as greases are widely used in a great variety of machines having rotating or sliding parts including industrial machines, automobiles, railway cars and the like with an object to reduce the friction and decrease wear of the parts. It is sometimes undertaken to admix a grease with a solid lubricant such as molybdenum sulfide with an object to improve the lubricant performance of the grease.
There is a problem, however, in the admixture of a grease with a solid lubricant such as molybdenum sulfide that, when such a grease is used in a machine part having a needle bearing in the structure such as a triport-type constant velocity joint, the solid component sometimes enter the needles. Therefore, such a grease with admixture of a solid ingredient is not always acceptable.
As an alternative way to improve the lubricant performance of a grease without using such a solid ingredient, the present inventors prepared a grease with a zinc dithiophosphate. This grease was tested. It was found that black denaturated matter is formed in the grease during use which enters the needles of a needle bearing so that regular rotation on the bearing is sometimes disturbed.